This invention relates to a system and method for acoustically detecting cutting tool breakage and rejecting background noise spikes that could cause false alarms, and more particularly, it relates to a circuit device and program logic which is able to differentiate between tool breakage and noise spikes caused by intermittent contact and non-contact resulting from machining of rough surfaces so as to avoid false alarms.
A machine tool monitor to detect broken tools and for part probing is described in commonly assigned U.S. Pat. Nos. 4,636,779; 4,636,780 and 4,707,687. A single sensor such as an accelerometer is mounted on the machine tool in a location with good coupling to vibrations generated at the tool-workpiece interface. The system is programmed to recognize signal patterns resulting from tool breakage. These patterns are typically abrupt, having substantial increases or decreases in the cutting noise mean signal level that persist for a given confirmation period and are caused by sudden changes in cutting conditions resulting from critical geometry changes in the cutting edge. Another signal pattern is evidenced by a gradual decrease in the cutting noise signal level due to a series of small breaks or other gradually occurring breakage.
A lathe tool break detection system has been developed that consists of a high frequency accelerometer to convert metal-cutting operation vibrations to broadband electrical signals, analog signal processing circuitry to amplify a selected band of signal frequencies and detect the energy in that band, and digital time-domain pattern-recognition logic to detect tool break signatures and reject normal cutting operation artifacts in the processed vibration signal. Pattern recognition logic detects tool break events occurring while machining the workpieces. This known tool break detection system has been successfully used for the detection of tool breaks in metal-cutting lathe operations in which the tools are various ceramic materials of round shape and the workpiece materials are tough aerospace alloys such as Inconel.
Tests with carbide tools cutting Inconel and other metals have shown the high frequency acoustic signal produced by a tool break event is often a dense high amplitude spiky noise. Moreover, both ceramic and carbide tools generate a great deal of spiky noise both at the beginning of a cut when the tool first makes contact with a workpiece and also during lathing operations, when the tool cuts a workpiece that has a rough surface so that the tool passes through air and workpiece material intermittently. This phenomenon is known as "runout condition", in which an initial cut on a rough surface causes the depth of cut to change abruptly one or more times per revolution of the part and the machine spindle. This situation can produce, under aggressive machining rates, very rapid increases and decreases in vibration signal level, including decreases to essentially zero level occurring when the tool passes completely out of the metal it is cutting.
Moreover, the vibrational frequency signatures generated by a runout condition and a tool break can be deceptively similar. Tool break signatures and runout noise bursts are not linearly separable on the basis of the amount, direction, or rate of the change of signal level produced. That is, for runout noise spike transients, the statistical spread of values for each of these measures alone, or in any combination, overlaps that of the corresponding measure for tool break signatures. Thus, a program logic is required which provides improved discrimination against runout vibration signal transient false alarms without adversely affecting the missed tool break detection rate realized by presently used program logic, and which is also usable with existing vibration based machine tool break detection systems, such as those described in U.S. Pat. Nos. 4,636,779, 4,636,780 and 4,642,617.